Huckabee, a Republican who leaves office in January, compared a flat tax to tithing.

"If you think about it, the biblical prescription of giving was a tithe, which meant you paid proportionate to your income. You paid 10 percent," Huckabee said on his monthly call-in radio show. "It was proportionate to each person, so there was a complete equity. I think the idea of a flat tax is exactly what we need."...